1. Field of the Invention
This invention relates to the migration of a molten zone through a solid body of semiconductor material by thermal gradient zone melting.
2. Description of the Prior Art
W. G. Pfann described in "Zone Melting", John Wiley and Sons, Inc., New York (1966), a thermal gradient zone melting process to produce various desirable material configurations in a body of semiconductor material. The process had previously been disclosed in his issued U.S. Pat. No. 2,813,048, based on his application filed June 24, 1954. In both instances, cavities are generally formed in the surface of the body and a piece of wire of the metal to be migrated is disposed on the cavity. However, the resulting structures were not desirable for semiconductor usage.
M. Blumenfeld, in U.S. Pat. No. 3,897,277, teaches alloying aluminum to the surface of the body of silicon semiconductor material in an attempt to maintain the registry of the pattern of metal deposits to be migrated. However, problems of precise registry of the metal still plague one's attempt to obtain the precision necessary to obtain an array of deep diodes suitable for making x-ray imaging devices.
Recently, T. R. Anthony and H. E. Cline, discovered that employing selective chemical etching of the surface and preferred crystallographic orientation of the surface and molten zones enabled one to employ thermal gradient zone melting processing to assist in making semiconductor devices commercially feasible. The improved process resulted in a large savings in energy required to process semiconductor materials and increased yields. For a further teaching of the improved process, one is directed to their teachings in their recently granted U.S. Pat. No. 3,904,442, and co-pending patent application Ser. No. 519,913 filed Nov. 1, 1974.
To practice thermal gradient zone melting on a commercial basis, one should make the process as simple as possible. Consequently, John Boah in his copending U.S. patent application Ser. No. 578,807, filed May 19, 1975, describes the use of radiant energy as a source for the migration or movement of a molten zone through a solid body. The process and apparatus arrangement taught by Boah is very practical for many devices. However, a temperature gradient exists laterally across the width or diameter of the major surface of the body exposed to the source of radiant energy.
We have found that non-uniformities in temperature distribution under the quartz lamps exist over distances corresponding to the lamp spacings and the tungsten disk holder spacings. But additionally we discovered monotonic fall-off in temperature occurred away from the center of the lamps because of the finite size of the lamp array. This temperature fall-off occurs for two reasons: First, the solid angle of illumination observed by the sample wafer is greater beneath the center of the lamp array than beneath the side of the lamp array. Second, a temperature fall-off toward the sides of the variation of reflectivity of quartz with the incident angle of light.
In quartz lamp radiant heaters, we are concerned with two layers of quartz and possibly three if we include an extra quartz convection suppressor. First, there is the quartz envelope of the quartz lamp. Secondly, there is the quartz coverplate of the air cooling channel around the quartz lamps. If one measures the transmission of quartz to light incident at an angle .theta. from the normal of a quartz plate, transmission is relatively constant up to about an angle .theta. of 40 degrees. Beyond this angle, transmission begins to decrease sharply until at 90.degree., no transmission occurs. A sample placed at the center of the lamp array sees more light incident on the overhead quartz channel plate at lower values of the incident angle .theta. on the average than a sample toward the side of the array of lamps.
If we use a centro-symmetric rotation under the center of the lamp array to achieve uniform migration of a molten zone there is a danger that radial migration distortions of the molten zone may still occur in the wafer. There are two reasons why we believe this distortion occurs. First, all parts of the wafer (center of rotation) are always at the same radial location from the center of the lamp array. Because of the radial fall-off in the temperature away from the center of the lamps, the molten zone during migration will tend to migrate toward the center of the lamp array on all parts of the wafer, thereby distorting the molten zone geometry.
It has been discovered that the lateral temperature gradient distorts the movement of the molten zone. That is the geometrical configuration of the regions produced by the movement of the molten zone through the solid body changes in accordance with the thermal gradients, both lateral and practical, in the region of movement. The lateral thermal gradient is the one of greatest concern. In forming grid structures, for electrical isolation of devices, distortion of the array to form the grid may be so great as to eliminate the outer 2 mm peripheral portion of a wafer from being usable. The geometrical pattern of the grid structure is very distorted. Separation of isolation regions occur and surface tension pulls intersecting regions apart during migration.
It is therefore an object of this invention to provide a new and improved thermal gradient zone melting process which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved temperature gradient zone melting process wherein any lateral and/or radial temperature gradient in a body or wafer of semiconductor material being processed thereby is minimized.
Other objects of this invention will, in part, be obvious and will, in part, appear hereafter.
In accordance with the teachings of this invention, there is provided a new and improved thermal gradient zone melting method for migrating a molten zone through a solid body of semiconductor material. The method comprises the process steps of selecting a body of single crystal semiconductor material having two major opposed surfaces which are, respectively, the top and bottom surfaces thereof. The body has a predetermined type conductivity, a predetermined level of resistivity, a preferred diamond crystal structure, a preferred planar crystal orientation for at least the top surface, and a first preferred crystal axis and a vertical axis which are each substantially perpendicular to the top surface and substantially parallel with each other. A layer of metal of a predetermined thickness and a predetermined geometrical configuration is preferably vapor deposited on the major surface having the preferred planar crystal orientation. The processed body is placed on a support and rotated simultaneously about the vertical axis of the support and about its own vertical axis.
The body and the deposited metal is heated to a preselected elevated temperature sufficient to form a melt of a metal-rich semiconductor material on the surface of the body while continuing the dual rotation of the body. A temperature gradient is established substantially parallel with the vertical axis of the body and the first preferred crystal axis of the crystal structure while continuing the dual rotation of the body. The surface on which the melt is formed is retained at the lower temperature. Thereafter, each melt of metal-rich semiconductor material is migrated as a molten zone through the solid body of semiconductor material for a sufficient period of time to reach a predetermined distance into the body from the surface on which the melt is formed. The dual rotation of the body is continued during this migration. A region of recrystallized semiconductor material of the body having solid solubility of the deposited metal therein is formed in the body by each melt. Each region so produced has a predetermined geometrical configuration, a substantially uniform cross-sectional area and a substantially uniform level of resistivity throughout the entire region.